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.\" Copyright (C) 2000, 2001 Internet Software Consortium.
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.\"     Title: lwres
.\"    Author: 
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\"      Date: 2007-06-18
.\"    Manual: BIND9
.\"    Source: ISC
.\"  Language: English
.\"
.TH "LWRES" "3" "2007\-06\-18" "ISC" "BIND9"
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.SH "NAME"
lwres \- introduction to the lightweight resolver library
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <lwres/lwres\&.h>
.fi
.ft
.SH "DESCRIPTION"
.PP
The BIND 9 lightweight resolver library is a simple, name service independent stub resolver library\&. It provides hostname\-to\-address and address\-to\-hostname lookup services to applications by transmitting lookup requests to a resolver daemon
\fBlwresd\fR
running on the local host\&. The resolver daemon performs the lookup using the DNS or possibly other name service protocols, and returns the results to the application through the library\&. The library and resolver daemon communicate using a simple UDP\-based protocol\&.
.SH "OVERVIEW"
.PP
The lwresd library implements multiple name service APIs\&. The standard
\fBgethostbyname()\fR,
\fBgethostbyaddr()\fR,
\fBgethostbyname_r()\fR,
\fBgethostbyaddr_r()\fR,
\fBgetaddrinfo()\fR,
\fBgetipnodebyname()\fR, and
\fBgetipnodebyaddr()\fR
functions are all supported\&. To allow the lwres library to coexist with system libraries that define functions of the same name, the library defines these functions with names prefixed by
lwres_\&. To define the standard names, applications must include the header file
<lwres/netdb\&.h>
which contains macro definitions mapping the standard function names into
lwres_
prefixed ones\&. Operating system vendors who integrate the lwres library into their base distributions should rename the functions in the library proper so that the renaming macros are not needed\&.
.PP
The library also provides a native API consisting of the functions
\fBlwres_getaddrsbyname()\fR
and
\fBlwres_getnamebyaddr()\fR\&. These may be called by applications that require more detailed control over the lookup process than the standard functions provide\&.
.PP
In addition to these name service independent address lookup functions, the library implements a new, experimental API for looking up arbitrary DNS resource records, using the
\fBlwres_getaddrsbyname()\fR
function\&.
.PP
Finally, there is a low\-level API for converting lookup requests and responses to and from raw lwres protocol packets\&. This API can be used by clients requiring nonblocking operation, and is also used when implementing the server side of the lwres protocol, for example in the
\fBlwresd\fR
resolver daemon\&. The use of this low\-level API in clients and servers is outlined in the following sections\&.
.SH "CLIENT-SIDE LOW-LEVEL API CALL FLOW"
.PP
When a client program wishes to make an lwres request using the native low\-level API, it typically performs the following sequence of actions\&.
.PP
(1) Allocate or use an existing
\fBlwres_packet_t\fR, called
\fIpkt\fR
below\&.
.PP
(2) Set
\fIpkt\&.recvlength\fR
to the maximum length we will accept\&. This is done so the receiver of our packets knows how large our receive buffer is\&. The "default" is a constant in
lwres\&.h:
\fBLWRES_RECVLENGTH = 4096\fR\&.
.PP
(3) Set
\fIpkt\&.serial\fR
to a unique serial number\&. This value is echoed back to the application by the remote server\&.
.PP
(4) Set
\fIpkt\&.pktflags\fR\&. Usually this is set to 0\&.
.PP
(5) Set
\fIpkt\&.result\fR
to 0\&.
.PP
(6) Call
\fBlwres_*request_render()\fR, or marshall in the data using the primitives such as
\fBlwres_packet_render()\fR
and storing the packet data\&.
.PP
(7) Transmit the resulting buffer\&.
.PP
(8) Call
\fBlwres_*response_parse()\fR
to parse any packets received\&.
.PP
(9) Verify that the opcode and serial match a request, and process the packet specific information contained in the body\&.
.SH "SERVER-SIDE LOW-LEVEL API CALL FLOW"
.PP
When implementing the server side of the lightweight resolver protocol using the lwres library, a sequence of actions like the following is typically involved in processing each request packet\&.
.PP
Note that the same
\fBlwres_packet_t\fR
is used in both the
\fB_parse()\fR
and
\fB_render()\fR
calls, with only a few modifications made to the packet header\*(Aqs contents between uses\&. This method is recommended as it keeps the serial, opcode, and other fields correct\&.
.PP
(1) When a packet is received, call
\fBlwres_*request_parse()\fR
to unmarshall it\&. This returns a
\fBlwres_packet_t\fR
(also called
\fIpkt\fR, below) as well as a data specific type, such as
\fBlwres_gabnrequest_t\fR\&.
.PP
(2) Process the request in the data specific type\&.
.PP
(3) Set the
\fIpkt\&.result\fR,
\fIpkt\&.recvlength\fR
as above\&. All other fields can be left untouched since they were filled in by the
\fB*_parse()\fR
call above\&. If using
\fBlwres_*response_render()\fR,
\fIpkt\&.pktflags\fR
will be set up properly\&. Otherwise, the
\fBLWRES_LWPACKETFLAG_RESPONSE\fR
bit should be set\&.
.PP
(4) Call the data specific rendering function, such as
\fBlwres_gabnresponse_render()\fR\&.
.PP
(5) Send the resulting packet to the client\&.
.PP
.SH "SEE ALSO"
.PP
\fBlwres_gethostent\fR(3),
\fBlwres_getipnode\fR(3),
\fBlwres_getnameinfo\fR(3),
\fBlwres_noop\fR(3),
\fBlwres_gabn\fR(3),
\fBlwres_gnba\fR(3),
\fBlwres_context\fR(3),
\fBlwres_config\fR(3),
\fBresolver\fR(5),
\fBlwresd\fR(8)\&.
.SH "AUTHOR"
.PP
\fBInternet Systems Consortium, Inc\&.\fR
.SH "COPYRIGHT"
.br
Copyright \(co 2004, 2005, 2007, 2014-2016 Internet Systems Consortium, Inc. ("ISC")
.br
Copyright \(co 2000, 2001 Internet Software Consortium.
.br
